.....at : http://mechanicalexpressions.com/explore/dynamics/trebuchet.html...if we asume that the length of the arm , from the fulcrum to the mass "m" , is oscillatory ("sliding" in some manner , to and fro along the arm ) so to play a continuous unbalance (torque difference) , on the same side of the fulcrum. It's only a small "step of thinking"... Al_ex